public static boolean esEmail(String correo)
{
    Pattern pat = null;
    Matcher mat = null;
    pat = Pattern.compile("^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,3})$");
    mat = pat.matcher(correo.trim());
    if (mat.find()) {
        return true;
    }else{
        return false;
    }
}

pat = Pattern.compile("^[_a-z0-9-]+(\.[_a-z0-9-]+)*@+(\sitio.com)$");